Asteroids and comets, metal and rocky objects, move in a vertiginous speed around planets and around our Sun. Meteorites and comets bombard our planet since the birth of the solar system. Although they seem wisely installed on their orbits between Mars and Jupiter, they are sometimes destructive and we owe them credibly the appearance of the life on Earth. Asteroids being our most close neighbors, they are somehow, bound to our fate. The total mass of all the asteroids of the main belt revolving around the Sun between Mars and Jupiter is estimated at 4×1021 kilograms, what amounts to approximately 5 % of the mass of the Moon. We distinguish various categories: silicates (group s), the carbon (group c) and the metal (group m). The first ones consisting of minerals, the second of carbon and ice and the last ones of metals. Asteroids are classified by groups according to their place within the belt of asteroids. We can quote the following groups: Hungarias, Floras, Phocaea, Koronis, Eos, Themis, Cybeles and Hildas which are named according to the main asteroid in the group).

Image: The asteroid (25143) Itokawa is like Apollo (near-Earth asteroids), the image was taken by the Japanese probe Hayabusa in 2005. Probe arrived near the asteroid Itokawa September 12, 2005.'s Hayabusa probe, the Japan Aerospace Exploration Agency (JAXA), landed on the asteroid 19 November 2005 to collect rock samples. Back on Earth (2010). Photo credit (agency JAXA).

Galileo probes, Near, Deep Space

It has been not enough time since the man investigates closely these asteroids. Indeed, given their fast movements, probes would have had difficulty in approaching it. It is in 1991, October 29th, that the Galileo probe, heading for Jupiter, approached for the first time the asteroid: Gaspra. The probe approached in 1 600 km, enough near to establish these dimensions in a precise way (20x12x11km, revealing 50 meters details. Almost two years later, on August 28th, 1993, Galileo met a second asteroid, Ida. Being then situated to 3 UA, they crossed themselves in km/s 12.4, separated one of the other one by 2400 km. This meeting revealed something very surprising, the presence of a satellite glancing through it as height as 100 km, baptized Dactyl.February 17th, 1996, on Cape Canaveral, sound NEAR (for " Near Earth Asteroid Rendezvous ") took off in the direction of the asteroid Eros, the purpose being to be on the orbit to study it under all the angles during one year. It glanced through in the passage Mathilde in 1200 km in height. After a first failure in 1999, the probe returned and was on the orbit on February 14th, 2000 to settle on February 12th, 2001. About at the same time, the Deep Space 1 probe was thrown in October, 1998 towards the asteroid Squall, by approaching as height as 15 km only, taking a multitude of photos.

The Cassini-Huygens probe, heading for Saturn, photographed as for her Masursky. The biggest asteroids known for the belt are Ceres, Pallas and Vesta. They are the only ones to have a average diameter superior to 500 km, thus the only ones to have an almost spherical geometry. Indeed, from a 500 km diameter, a rocky celestial body can take a spherical shape. The mass of the celestial body is sufficient to exercise a strength of gravitation superior to the strengths of cohesion of the matter. The matter divides up thus naturally into ball under the effect of its own weight. Below 500 km in diameter, asteroids as Gaspra (15 km) or Ida (56 km), present much more irregular forms. More solar system objects are small (asteroids, comets) and more they interest the scientists because they spread all their energy into space and kept intact materials, of the time of their "death", particularly organic molecules.

Image: largest known asteroids (above).

Asteroids

Approximatedimensions

Discovery date

Ceres 1

974.6 km

1801

Pallas 2

582×556×500 km

1802

Vesta 4

572.6x557.2x446 km

1807

Hygiea 10

530x407x370 km

1849

Sylvia 87

384x262x232 km

1866

Hektor 624

370x195x195 km

1907

Europa 52

360x315x240 km

1858

Eunomia 15

357x355x212 km

1851

Davida 511

357x294x231 km

1903

Interamnia 704

350.3x303.6 km

1910

Camilla 107

344x246x205 km

1868

Juno 3

320x267x200 km

1804

Cybele 65

302x290x232 km

1861

Hermione 121

268x186x183 km

1872

Euphrosyne 31

255.9 km

1854

Chariklo 10199

248x258 km

1997

Iris 7

240x200x200 km

1847

Psyche 16

240x185x145 km

1852

Daphne 41

239x183x153 km

1856

Kalliope 22

235x144x124 km

1852

Amphitrite 29

233x212x193 km

1854

Zones with asteroids

Between Mars and Jupiter (from 300 to 600 million km of the Sun).

Asteroids are mainly situated in the main belt, between the orbits of Mars and Jupiter. We find several hundreds of thousand listed objects there. All its objects would have been able to form a planet in this zone but the gravitational disturbances of Jupiter did not allow it. Jupiter played defender's role of the life on our planet. Without Jupiter, the bombardment on Earth would be 1000 times as frequent.

In the belt of Kuiper (between 35 and 100 UA of the Sun).

This zone contains ice-cold objects, and are not thus strictly speaking asteroids. This belt is rather a crèche of comets. The first member is discovered in 1992. We count a little more than 1000 today. The British call the asteroids of this type of "cubewanos". The biggest identified until today is Quaoar (1280 km in diameter).

In the cloud of Oort (between 20000 and 150000 UA neighborhood).

This zone of the sky, the remainder of the original nebula, would contain billions of nuclei of comets and would be the source of most of the new comets which enter the central regions of the solar system. The Cloud of Oort constitutes the reservoir of comets in long period. It is one diameter 1000 time superior in that of the solar system that we know with the 8 planets.

Image: Ceres view from the Hubble Space Telescope (ACS). The contrasts were raised to reveal the details of the surface. Credit NASA, ESA

Ceres

Characteristics

Aphelion

445 280 000 km

Perihelion

382 520 000 km

Semi-major axis

413 910 000 km

Equatorial diameter

974.6 km

Polar diameter

909.4 km

Axial tilt

3°

Mass

9.445×1020 kg

Average orbital speed

17.882 km/s

Eccentricity

0.075797

Orbital period

4.6 yr or 1680.99 d

Synodic period

1.278 yr or 466.7 d

Inclination to Ecliptic

10.593°

Longitude of ascending node

80.3276°

Argument of perihelion

72.2921°

Right ascension

291°

Pallas (2)

Pallas was discovered March 28, 1802 in Bremen by the German astronomer Heinrich Wilhelm Olbers (1758-1840) while he took care of observations to locate and determine the orbit of the asteroid Ceres, discovered the previous year by Giuseppe Piazzi (1746/1826), the average forecast of the great mathematician Carl Friedrich Gauss (1777-1855). Pallas is the second largest asteroid in the main belt (526 km mean diameter) and the second which was discovered. The object is according to U.S. researchers, a protoplanet, i.e. a body massive enough to form a planet, but frozen in its evolution by lack of materials. Pallas was named in honor of the Greek goddess of wisdom, science and arts.

Image: With a diameter of 570 x 525 x 482 km, Pallas was the second largest known asteroid belonging to the asteroid belt between Mars and Jupiter. Pallas running on a highly inclined orbit of 34.8 degrees. Its orbital period is 4.6 years. With an eccentricity of 0.23, its distance varies from 2.14 AU at closer to the Sun and 3.41 AU away. High resolution images of the Hubble Space Telescope, we present here a model of an asteroid almost round.

Juno (3)

Juno is the third asteroid discovered in the Solar system, Juno ( 3 Juno) pull its name of the woman of Jupiter. Juno, is discovered by Carl Ludwig Harding to the look-out observatory of Lilienthal, near Bremen, on September 1st, 1804 by means of a simple telescope of 5 cm of opening. The orbit of Juno changed in 1839, maybe because of an impact. Images realized in adaptive optics in the look-out observatory of the mountain Wilson (United States) reveal that the small planet 240 km in diameter was totally deformed by a collision which created an immense crater of a size of 100 km looking like a bite.A system of adaptive optics allowed to obtain an outstandingly clear sight of Juno, by reducing the interferences with the ground atmosphere. (Sallie Baliunas and al.)

Juno

Characteristics

Dimensions

320x267x200 km

Mean radius

≈233 km

Mass

2.82±0,12×1019 kg

Rotation period

7.21 hr

Temperature

≈163 K

Aphelion

502.08×106 km

Perihelion

328.87×106 km

Semi-major axis

399.48×106 km

Axial tilt

51°

Orbital period

4.37 a or 1595.4 d

Average orbital speed

17.93 km/s

Eccentricity

0.2568

Inclination to Ecliptic

12.968°

Longitude of ascending node

169.96°

Argument of perihelion

247.93°

Image: This representation shows the track on the asteroid Juno made by an impact. (David A. Aguilar, Harvard-Smithsonian Center for Astrophysics).

Vesta (4)

Vesta, the asteroid the most glittering with all, carries the name of the Roman goddess of the health. It is the only visible asteroid in the bare eye. Discovered on March 29th, 1807 by Heinrich Olbers, Vesta is the fourth " telluric planet " to be discovered. Among asteroids, it is the second en masse and the third in size. It turns around the Sun in 3,6 ground years and its mean diameter is about 520 km. The composition of its surface is of basalt nature. Vesta possesses an enormous crater near its south pole. Vesta is thus a big rock of irregular shape, without track of water with a nucleus formed by iron. The enormous crater in Vesta's south pole (460 km wide and 13 km deep) is the result of a collision, The astronomers think that 5 % of meteorites found on Earth result from this gigantic shock.

Image: The image opposite shows Vesta seen by the spatial telescope Hubble. Vesta, with its 520 km in diameters average is much more massive than any other body of this region of the belt of asteroids.

Vesta

Dimensions

578 × 560 × 458 km

Mass

2,7 × 1020 kg

Mean density

3,4 g/cm3

Rotation period

5,342 h

Image: Vesta should be visited by the mission of the NASA Dawn left for 2007.

Gaspra (951)

Gaspra discovered in 1916 by Grigoriy N. Neujamin and observed from 1991 till 1993 by Galileo during its journey towards Jupiter. It was the first meeting moved closer with a planetoid. Its surface (20x12x11 km) reflects 20 % of the solar light. Gaspra was glanced through at a 1600 km height. The Galileo probe activated its cameras 7 hours before this historic meeting and collected details of hardly 50 meters, so confirming that it was about an asteroid of type S, consisted of rocks, iron and common chondrites. The asteroid Gaspra, is not the one witch worries most the scientists, it is not a part of potential earth-cruisers.

Image: This image is built from two photos of Gaspra realized by the probe Galileo through clear filters, since a distance of 5300 km, on October 29th, 1991 and 10 minutes, before the passage in the closest point. It presents Gaspra with the most high resolution (approximately 50 m by pixel). More of 600 craters of more than 100 m of diameter can be seen. The visible portion here is approximately 18 km in its biggest width. Gaspra is a body of irregular shape about 19 x 12 x 11 km. The north pole is on the upper left. Gaspra makes a tour on itself counterclockwise in 7 hours. credit: NASA

Ida (243)

Ida (name of a nymph of the Crete) was discovered by the Austrian astronomer Johann Palisa on September 29th, 1884. Ida is in the main belt of asteroids, between March and Jupiter, and is a part of the family of Koronis. With the asteroid Gaspra, Ida was observed in August, 1993 by the probe Galileo during its journey towards Jupiter. The probe showed that Ida possesses a small satellite. This one is named S / 1993 243 ) 1 Dactyl. It was the first discovery of a natural satellite around an asteroid. Ida is a body of shape irregular, placed by the scientists in the class S (looking like rocky or ferro-rocky meteorites).

It is a member of the family Koronis, supposed to be fragments of the explosion of an asteroid during a collision.The asteroid Ida and its satellite Dactyl, photographed in 1994 by the probe Galileo at a distance of 10870 kilometers. Ida (to the left) has a 56 kilometer dimension and Dactyl (the point to the right) is 1.5 kilometers. Credit: NASA

Mathilde (253)

Mathilde is the asteroid N ° 253. While examining photos sent by the probe NEAR a team of the University of Washington in emitted a surprising hypothesis: the asteroid Mathilde, whose diameter does not exceed 57 km, would be an enormous pumice, of a porosity of more than 50 %. This deduction is pulled by the study of the neighborhoods of three Mathilde's bigger craters. Seen their diameter (33,4 km for the biggest and 29 km for the others), the original impact has of the very violent being. However, we recover anybody ejected, these faces which usually testify of the pulverizing then the ascent of matter during the shocks. Now, in spite of a very weak gravity (a thousandth of that of the Earth), Mathilde should present of such structures.

It is for it that the scientists think that the asteroid has the consistency of a toweling. It cashes then the knocks by becoming more and more compact. The astronomers of Washington suggest that the bodies which surrounded the young Sun had to possess the same structure as that of Mathilde. Image of the asteroid ( 253 ) Mathilde taken at a distance of 1200 km by the probe NEAR after its passage in the closest point on June 27th, 1997. Craters of impact going of 30 km unless 0,5 km are visible. We consider that the dimensions of Mathilde make 50x53x57 km. 60 % of the surface was photographed by NEAR and five craters are more than 20 km in diameter.

Eros (433)

433 Eros is an S-type asteroid in the family Amors discovered August 13, 1898 by the German astronomer Gustav Witt, the first of this family to be discovered. The spacecraft NEAR Shoemaker (Near Earth Asteroid Rendezvous), launched February 17, 1996 by NASA, landed February 12, 2001, engineers at the control center managed the feat to put NEAR on the surface of the asteroid Eros to make a complete mapping of the surface of Eros. The high-resolution images of its surface has been combined with measurements taken by the NEAR laser range finder to get the image to the cons. Eros is a solid one piece to almost uniform composition was formed during the early formative years of our solar system. The probe, when it had not been scheduled to land on the asteroid, has survived long enough to provide an analysis of the composition of the dust covering the asteroid. In December 2002, NASA was unable to reestablish radio contact with the probe.